Outer steel structures using thin metallic sheets such as a steel sheet, for durable consumer goods such as automobiles, building materials, furniture, electrical appliances, etc, have been again examined in recent years because they are advantageous from the aspects of recycling properties, removal of detrimental substances in processing of the wastes, and ecology. When means for reducing the thickness of a metallic external sheet by supporting the external sheet by reinforcing beams so as to reduce the weight is employed, tapping the metallic external sheet is likely to spoil the commercial value of products since the metallic external sheet is generally as thin as about 1 mm. In other words, when such a thin metallic sheet is tapped or patted, a sound inherent to the metal (the sound is an offensive and low-grade noise because it is not heavy and low) occurs, and the value of the product itself might be spoiled by this noise. Particularly in the case of products for home use, tone quality and quietness of the tapping sound of the external plate, which is generated for some reason or other, is one of the important factors for providing a feeling of high quality to the product.
A sheet material produced by sandwiching a resin film between thin metallic sheets and called a "vibration-proofing steel sheet" has been used in the past as the metallic external sheet satisfying the object described above. When this material is used, sound damping characteristics can be remarkably improved, but the material is not free from the problems that the cost is high, machining performance such as bendability, drawability, etc, is inferior to a single sheet because it is a sandwiched metallic sheet, and sharp bending and drawing cannot be carried out.
Japanese Unexamined Patent Publication (Kokai) No. 62-27143 discloses a composite vibration-proofing steel sheet which can withstand bend-machining at a small bending radius by simultaneously heating and shaping a metallic sheet and a vibration-proofing material without using an adhesive, and Japanese Examined Utility Model Publication (Kokoku) No. 6-49743 describes a steel sheet for a floor produced by the steps of applying a polymer resin to a part of a bent steel sheet to obtain a thin steel sheet and bonding the thin steel sheets by a polymer resin so as to damp a sound.
These proposals are based on the premise that the thin steel sheets and a plastic material as the vibration-proofing material are finally bound together. When the structure requiring an adhesive is employed, the increase of the cost of the adhesive and the drying time render the disadvantages for the production, and deterioration of the adhesive might occur depending on temperature and the passage of time. In the case of electric home appliances, in particular, electrical components inside the casing exist under exothermic condition, and counter-measures for thermal degradation of the adhesive and for possible fire hazards become necessary.
Further, Japanese Unexamined Patent Publication (Kokai) No. 61-182820 discloses an invention wherein the ratio of thickness of three layered steel sheets is selected within a specific range and these steel sheets are locally fused so as to improve vibration-proofing performance by utilizing the friction between the metallic sheets. However, the use of the laminated steel sheets is not preferable for those products whose weight must be reduced, and various problems are yet left unsolved for applying this invention to products for which machinability and appearance are of importance.